Ceramic circuit boards often need to be electrically contactable from both sides and to allow for through-plating through the circuit board. In general, this is carried out by means of holes that are provided in the circuit board and are filled with an electrically conductive material. Such vias normally have diameters of approximately 100-300 μm.
What is nowadays usually used as an electrically conductive material are comparatively expensive silver or other precious metal pastes which are composed of one or more metal powder(s), optionally an adhesive glass proportion of 1-10% by weight, for example PbO, B2O3, Bi2O3 or SiO2, and a high-boiling organic matter, comprising binders such as ethyl cellulose or polyvinyl butyral, and solvents such as Terpineol or Texanol. If the substrate material for the circuit boards consists of AlN, then ZnO, SiO2, CaO, TiO2 and B2O3 may be used as adhesive glasses.
Copper metallisations are also increasingly used as electrically conductive materials, wherein pastes filled with copper particles having diameters of approximately 1-10 μm and an adhesive glass proportion are often used for filling the vias. Subsequently, the material is burned in at 650-1200° C. in a nitrogen atmosphere having a low oxygen content (<1-100 ppm). If copper pastes are used, the problem arises that they greatly shrink, crack and/or fall back out of the vias during burning-in if they only consist of the usual substances mentioned above. In this case, large cavities and/or cracks may occur in the metallisation or in the binding region of the full-area metallisation. In the worst case, the vias may sinter together to form a pin and fall out of the hole. In cases in which cracks or cavities occur, the vias have an elevated electrical resistance and are not hermetically sealed either.
Tightness against the surrounding atmosphere is particularly important if oxygen-sensitive parts such as circuits (Si), LEDs or OLEDs are to be integrated. Towards the top, these components are usually protected by covers, lenses or the like, which are soldered or glued on. However, also the bottom has to be protected against the ingress of air or humidity, which is realised, inter alia, by means of hermetically sealed metallisations of the vias.
Attempts to counteract shrinking of the electrically conductive material during sintering are made by adding adhesive glass. Despite the adhesive glass contained in the paste, however, the negative effects of shrinking can often at least not sufficiently be avoided.
This phenomenon is a particular problem in the case of AlN ceramics because only few substances do not cause the decomposition of the AlN in heat. AlN is only metastable and decomposes forming the lowest energy aluminium compound Al2O3 under certain conditions. Some oxides, such as in particular the Bi2O3 that is often used, or the PbO that used to be applied, vehemently attack AlN, decomposing same to form aluminium oxide, lead and nitrogen according to the following reaction equation:2AlN+3PbO→Al2O3+3Pb+N2.
Other oxides such as ZnO decompose AlN only slowly, and therefore they are better suited as binders of the metallisation to ceramics. In addition, it is important in the case of AlN to achieve a low TCE (thermal coefficient of expansion) such that the compound does not crack.
Moreover, conventional additives such as lead oxide or bismuth oxide are to be avoided for ecological reasons, or they react too strongly with AlN. The nitrogen that is formed from the AlN as a result of the decomposition reaction, see above, makes the metallisation porous and causes it to swell.